Electron beam controlling apparatus



June 28, 1960 ELECTRON BEAM CONTROLLING APPARATUS Filed Dec. 8, 1954 new mw/vr swim B. R. CLAY ETAL 3 Sheets-Sheet 1 2y MU Q Irma/vi) Filed D90. 8, 1954 June 28, 1960 B. R. CLAY ETAL 2,943,218

ELECTRON BEAM CONTROLLING APPARATUS ill-5 0/ x 7/ 10 )2 f 66 667 [5 V NTORJ Every/v 51/9) WW M June 28, 1960 B; R. CLAY ETAL ELECTRON BEAM CONTROLLING APPARATUS 3 Sheets-Sheet 3 Filed Dec. 8, 1954 INVENTO I .BZ/HU/YE (27/ Kari/YE hem m0 ,Wratwn ELECTRON BEAM CONTROLLING APPARATUS Burton R. Clay, Woodbury, and L'oren Rt Kirkwood, Haddonfield, N.J., asslgnors to Radio Corporation of America, a corporation of Delaware Filed Dec. 8, 1954, Ser. No. 473,855

12 Claims. (Cl. 313 -77) The present invention relates to new and improved apparatus 'for use in controlling electrons in cathode ray tubes of the type employed as color television image reproducing devices. Specifically, the invention relates to such apparatus which is adapted for use in conjunction with cathode ray tubes having a plane of deflection at which electrons are subjected to a scanning movement in their travel toward a screen unit of the type comprising a so-called mosaic screen and one or more adjacent grills or mask-s through which electrons pass in different angular directions to preselected elemental areas of the mosaic.

While the present invention will be described herein as it may be applied to a cathode ray tube of the dotscreen variety disclosed in an artiicl'e by H. B. Law A Three-Gun Shadow-Mask Color Kinescope (October 1951 issue of Proceedings of the IRE), its applicability is not so limited, since the invention :may also be employed with other types of cathode ray tubes wherein the angle of approach of an electron beam toward amosaic screen determines its point of contact with the screen.

As described in the cited Lawarticl'e, the particular screen area which is illuminated at any given instant in a cathode ray tube of the type in question a function of the precise angle at'which the electron beam approaches the color screen. When such tubes are manufactured in accordance With present-day mass production methods, it is not always possible to maintain the necessary accuracy in the assembly of the grill orrnas'k "the hosphor screen. By virtue of such -diificulties, -many cathode ray tubes which are otherwise satisfactory in structure must be rejected because of color dilution. One particular form of color dilution which is encountered in color kinescopes is that which results from misalignment of the shadow mask or grill about its longitudinal axis with respect to the phosphor screen, this form of color dilution being termed tangential or 'rorational color dilution. Another term of color dilution is that which .results from radial misalignment of shadow or grill "and screen such that the electrons strike a point on the screen radially displaced from their desired point of impingement and which may be termed radial color dilution.

Still another source of ditficulty in the operation of color kinescopes of the type under consideration is that which is brought about by stray magnetic fields such, for example, as the "earths magnetic field and which may be .tangential and/ or radialin nature.

It is, therefore, a primary object of-the present invention to provide new and improved apparatus for ipreventing color dilution of the type stemming from rotational and/ or radial misalignment of the tubeparts and/orsfrom magnetic fields.

Insofar as the correction for color dilution resulting from rotational misalignment in thescreen unit of a colorqkinescope is concerned, one satisfactory proposal made heretofore is that which 'provides an electromagnetic coil of suitable internal diameter for permitting itslocanitcd States Patent tion around and in general alignment with the screen unit of a cathode ray tube. Passage through the coil of a suitable direct current produces a magnetic fieldin the vicinity of the screen unit for correction of tangential mislocation of an electron beam with respect to the mosaic phosphor screen.

It has been determined,however, that certain manufacturing misalignment errors and those attributable to stray fields affect beam positioning differently in different quadrantsof the scanned raster, so that a single coil may be incapable of effecting rotational correction in such a case. 'That is to say, different corners of the raster may require difierent correctional treatment. Hence it is an object herein to provide correctional apparatus as described, which apparatus is capable of selective correction at different raster locations. I V

The matter of minimizing or eliminating color dilution which may be brought about by ambient magnetic fields has also been treated by the provision of a shield of magnetic material surrounding the electron beam ,pat'h within the kinescope between substantially its plane of deflection and screen unit. The present applicant has found that, while such a shield of magneticmaterial is eifective in protecting electrons Within the tube against magnetic field components in a plane transverse of the longitudinal axis of the cathode ray tube, magnetic field components parallel to such longitudinal axes are 'free to enter the electron beam path and to produce undesirable rotation of the electronstherein.

It is another object of the present invention to provide novel "and improved neutralizing apparatus for substantially precluding interference with electron beam paths within a cathode ray tube image device from magnetic field components including those components parallel to the longitudinal axis of the tube and components transverse of the axis. 7

In general, the present invention provides means for subjecting the electron beams, in the space between the tubes shadow mask and target screen to substantially constant direct current magnetic fields of such intensity, polarity "and orientation as to direct said 'e'lectronsto their normal or intended points of impact on the screen, regardless of the section of the screen in whichft he beams may be'operating. in accordance with a specific illustrative embodiment of the present invention, such direct current, magnetic field producing means may coniprise a plurality of permanent magnets having north and south poles and disposed around the screen unit of the cathode ray tube, each of the magnets being in association with magnetic pole forming means which may comprise a pair of rings of iron or the like, spaced from each other axially of the tube and discontinuous in 'theregion of the magnet. Thus, rotationof each magnet about its axis -(i.'e., about an axis radial of the tube) aflbrds selection of axial or transverse magnetic fields, the degree of activity of which is extended over an angledeterm'ined by the 'arcuate length of the sectionalized pole-forming rings. Further in accordance with the illustrative form 'of the invention, "each magnet may be provided with a magnetic shunt, the permanent magnet and itsshun-t being mounted for relative movement in such manner that the strength as well as the direction of the magnetic fields produced by the magnets may be suitably adjusted.

Thus, and as will be more fully appreciated hereinafter, the magnets and their pole pieces maybe selectively oriented to produce reversible axial orytransverse fields. Such adjustable fields may serve to correct :tor either rotational or radial misalignment of the parts of the tube or to neutralize undesired stray fields.

Additional objects and :advantages of the present -invention will become apparent to those skilled-int'h'e art of the screen of the tube of Fig. 1;

Fig. 4 is a front view of the apparatus of Fig. 1; Fig. 5 is an enlarged detail view of a portion of the apparatus of Figures 1 and 4;

"Fig. 6 is a plan view of the apparatus of Figure 5; and

Figures 7 and 8 illustrate certain magnetic fields. Referring to Fig. l of the drawing, the color kinescope "10 shown therein comprises an evacuated envelope having a cylindrical neck portion 12 of glass, for example, which terminates in a flared cone portion 14 whose larger end is closed by a glass face plate 16 through which is visible the phosphor screen 18 of the target structure of the tube which further includes a shadow .mask or aperture mask electrode 20. The tube as described thus far may be of the type disclosed in the above'cited Law article in which the phosphor screen 18 is of the well-known dot screen variety. As shown in the drawing, the phosphor screen is deposited directly .upon the rear surface of the face plate 16 and the mask 20 is curved concentrically with the curvature of the face plate. Specifically, the screen 13 is provided on its .rear

'surface with a multiplicity of groups of red, blue and greenphosphor dots, thedots of each group. being arranged at the apices of an equilateral triangle. The mask element 20 of the screen unit comprises a thin metal plate containing a multiplicityof apertures arranged in the same triangular pattern as the trios of phosphor dots such that there is onemask aperture for each trio of phosphor dots. The aperture mask 20 is supported in spaced relationship with respect to the screen 18 by suitable means (not shown).

The cylindrical neck portion -12 of the kinescope 10 houses three electron guns 24, 26 and 28, each of which produces an electron beam intended for bombardment of a particular screen color. The guns 24,- 26 and 28 maybe arranged at the apices of an equilateral triangle as shown in the Law article or in any other suitable manner such, for example, as an in-line arrangement. The electron beams produced by the guns are indicated diagrammatically by the dotted lines 30, 32 and 34 and are ,focused in a conventional manner by suitable means indicated as an electromagnetic focus coil 36 energized by currents from a source 38, whereby to provide fine spots at the target screen. The electron beams are subjected to the action of substantially perpendicular magnetic fields for the scanning, in horizontal and vertical directions, of a conventional rectangular raster at the screen unit. Such-scanning fields are produced by means of a deflection yoke 40 which may comprise a pair of normally, arranged deflection windings disposed about the neck of the kinescope and energized by suitable. sawtooth currents of television line and field frequencies from the deflection circuits 42. As indicated by the dotted line 44-44, the plane of deflection for the .three beams 30, 32 and 34 extends transversely through the deflection yoke 40.

Iii subsequent portions of the instant specification, reference will be made to the X, Y and Z axes of the kinescope 10. In order to facilitate an understand, ing of such designations, Fig. 2 illustrates, in simplified form, the kinescope 10 oriented about its several axes X, Y and Z. It will be seen from Fig. 2 that the netic fields (axial or transverse, for tangential orradial .errors, respectively), in their travel between the shadow .mask and the phosphor screen 18. The intensity and Z-axis coincides with the longitudinal axis of the tube,

while the X and Y-axes are normal to each other and to the -Z -axis.

As has been stated generally supra, one object of the present invention is that of eliminating the so-called tangential color dilution which results from a situation tantamount to that which exists when the shadow mask of a screen unit is rotationally displaced with respect to the phosphor screen (about their common axis). Fig. 3 illustrates a front view of such a screen unit, showing a typical case of tangential dilution. It will be understood that the three beams 30, 32 and 34 are intended to converge at the shadow mask 20 and diverge therefrom so that the red beam 30 strikes the red-designated phosphor R and the other beams 32 and 34 strike the green and blue'designated phosphors G and B, respectively, which phosphor dots are arranged, as explained, at the apices of an equilateral triangle. Assuming that there is some rotational mislocation of the shadow mask 20 and screen 18 or that there exists a magnetic field in the path of the beams between the shadow mask and phosphor. screen unit such that components of the magnetic field are parallel to the Z" axis of the tube, color dilution of the tangential type will result. Thus, referring to Fig. 3 wherein it is assumed, for purposes of simplicity of description, that only the red beam 30 is on" and that only the red screen dots R" are intended to be struck by electrons, the tangential color dilution is manifest at peripheral regions of the screen such that the red beam spots are not centered exactly on the red phosphor dots. Rather the red beam spots are tangent to or overlap and, hence, illuminate peripheral portions of the adjacent blue and green phosphor dots, thus diluting the red lightjand preventing it from appearing with its proper degree of saturation.

-While notillustrated, in the drawing, it will be understood that radial color dilution, such as may result from a radial misalignment between the shadow mask 20 and screen 18, would be manifested by the beam spots being shifted laterally (i.e., horizontally or vertically) from its desired point of impingement, and that tially minimizes both forms of color dilution through the agency of means for subjecting the several electron beams to the action of substantially constant (i.e., D.C.) magpolarity of the field are chosen so as to divert the electrons fromtheir predetermined angularly related paths to other angular directions as required to direct them to their intended points of impact upon the phosphor screen Figure 1 illustrates means, in accordance with a specific form of the invention, for providing the requisite axial or transverse magnetic field, such means comprising a plurality of permanent magnets 50 disposed around the screen unit of the tube 10, as illustrated in Figure 4. In-

'-terposed between the magnets 50 and the cathode ray tube are some of the sectionalized or discontinuous iron rings 52 and 54 which serve as magnetic pole forming means. The pole rings 52 and 54 are held in place with respectto th'e'tube and with respect to each other by means of a' strip '56 of brass or aluminum or other nonmagnetic material, the rings being riveted or otherwise secured to the strap. For ease of mounting the assembly,

-the strap is split and its loose ends are provided with apertures 58 throughwhich a brass bolt 60 is passed for securing the assembly. 7

As shown further in Figure 4, the rings are not necessarily mounted directly on the tube 10 but may, rather,

.manent :magnets 50 are shown in Figure 4, that is, two for each corner of the raster 64 scanned by the tube, it will be understood that fewer or more magnets may be employed, depending upon the extent of-correc'tion control-"desired.

Referring now to Figs. 5 and 6, there is shown in enlarged fashion one of the permanent magnets of Figs. 1 and 4. The permanent magnet 50 is illustrated as a cylindrical magnet magnetized along a diameter and having :north and south poles designated N and S. The magnet is supported ina suspended fashion with respect to the sectional ized pole rings in the following'manner: a-ir ivet 6"6 is passed through an aperture 68 at the center of the magnet and a threaded bolt 70 is secured to the 't'op'of the rivet 66 as by means of a solder joint'72. The bolt is slidably received by a bushing 74 which is securelyheldan aperture 76 located centrally of a supporting plate 78. Secured to the end of the bolt 70 remote "from the magnet-50, as by means of a screw 80, is a knob 82. The bushing 74 has an axial, cylindrical 'extension 84 which is slotted throughout a portion of its periphery as at 86. A generally "U-shaped friction spring 88 which surrounds the sleeve portion 84 of the bushing communicates with the bolt 70 through the slot '86 and engages the bolt. Thus, as will be understood, the spring '88 is in engagement with the threads of the bolt 70, so that, when the bolt is rotated, it will thread its Way toward or away from the plate 78. Alternatively, the bolt may be pulled or pushed through the bushing 74 along its axis, which movement is permitted by the expansion of the spring 88 asit is cammed outwardly by the threads.

Secured to the underneath surface of the plate 78, as by spot welds, .for example, is a ring 90 of iron or other magnetic material and of internal diameter suficient to receive the magnet 50. Of the assembly thus far described, all of the members with the exception of the magnet, ring 90 and pole rings 52 and 54 may be of any suitable non-magnetic material. The plate 78 is supported above the pole forming rings as by means of outwardly 'bent cars 92 struck up from the supporting strap 56.

As has been stated, the pole forming rings in accordance with the invention are .not continuous about the periphery of the screen region of the tube but are, rather, sectionalized; In other words, as shown in Fig. 4, the pole ring 52 .is sectionalized into segments 52, 52a, 52b, 52c, 52d, 52c, 521, 52g, 52h and 521, while the pole-ring 54 is divided into the same number of segments matching the illustrated segments. In this manner, pairs of pole pieces 52, 54 and 52a, 54a, for example, are formed.

.Each magnet 50 operates in conjunction with the adjacent segments of two pairs of pole pieces. That is, the magnet 50 in Fig. 6 will be understood as acting in cooperation with the segments 52 and '54 forming one pair of 'pole'pieces and with the segments 52a and 54a forming an adjacent pair of pole pieces. As will further be seen from Fig. 6,, the extremities of the segments are or may be turned inwardly toward the center of the magnet 50 and are :spaced from each other so as to form a common air .gap in the region of the magnet. The same structure is present at the location of each of the other magnets and, as has been mentioned, there are four such operating positions or air "gaps on each aided the raster, although the number may be increased or decreas'edas necessary. By virtue of the plurality of magnets and the sectionalizedpole pieces, the operation of each magnet :regionalized, the extent of the region being a function of the arcuate length of-the pole segments, and yet not so strictly localized as would be the case in the absence of the elongated pole segments.

Since" all of the magnets are identical, only one need be described. in detail. From the foregoing, it will be understood that the'magnet '50 may be rotated about the axis of the bolt '70, in which event the spring 88 permits threaded movement of the bolt, and may also be moved axially toward or away from the pole pieces. Whenfthe magnet is in its extreme retracted position so that' it is within the ring '90, the ring acts as a shunt therefor, shortcircuiting the flux from the magnet so that the fiel'cl of the magnet outside of the ring is substantially zero. -On the other hand, 'when the magnet '50is as close as possible to the pole jpiece segments, the strength of the field produced by the magnet and passed through the pole piece segments is at its maximum value. For positions of the magnet intermediate the two described extreme positions, proportionately different field strengths maybe obtained. g

It is further to be noted that when the magnet 50 is oriented with its north and south poles aligned as "shown diagrammatically in Fig. 7 along a line parallel to the. pole piece segments, the segments 52a and'54a will serve as a pair of north pole forming pieces, whileithe seg ments 52 and 5 3 will serve as south poleforming'pieees so that the resultant 'flux lines are transverse *of the-tube 14 to produce radial deflection of electrons in the magnetic field. Conversely, when the magnet is rotated through '90 so that its poles are on a line perpendicular to thesegments 52 and '54, as in Fig. 8, the segments '54 andfi la will serve as north poles and the segments 52 and 52a as south poles, whereby the flux lines will ;be

axial of the tube and capable of producingtangenti'al movement of electrons within the region of their activity.

In the operation of the apparatus, it maybe assumed for purposes of illustration that tangential color dilution exists in one or'm'ore of the corners of the raster as illustrated by the showing of Fig. 3. In such event one or more of the magnetsstl in that region, may be rotated about its axis "so that it is positioned asshown in :Fig. 8. The resultant field may be varied in strength by moving the magnet towardlits associated "shunt 'ring 8 correction is required there, the permanent magnet nearest that section may be 'eifectively immobilized by retraction within the shunt ring 90. I

in a similar manner, radial color dilution in a given region of the raster may be corrected by rotating the magnet adjacent to that section so thatit is rpolarized as shown in Fig. 7. The resultant flux lines, the strength of which may be varied by movement *of the magnet axially of its bolt 70, will produce radialdefle'ction of the electron beams. The direction of deflection (i.'e., toward or away from the center of the raster) will be a function of the polarization of the magnet, as will be understood.

It should additionally be noted that two or more adjacent magnets may be caused to aid each other for the correct-ion of either tangential or radial error by polariz- .ing the magnets in the same sense.

Conversely, where opposite types or error exist in regions, the magnets :may be oriented for the necessary kind of correction without dele'teriously afiecting each other. In this connection it will be appreciated that the number ofmagnets and asso-' ciated pole ,piece segments may be increased where itis found that errors of the radial and tangential types occur alternately in adjacent segments of the raster.

Since the specific materials of which the magnets and the pole segments are formed do not constitute a part of the present invention, it is suflieient 'to note that the magnets may be of any suitable material such as Alnijco and the pole segments may be of soft iron.

From the foregoing, it 'willbe recognized that the present invention provides a simple but efiective arrange ment for selective correction of either tangential or radial mispositioning of electrons with respect to a target. Such selective action stems from the fact that the pole forming rings are sectionalized or segmented so that a given magnet may cause axially separated segments to act as opposite magnetic poles (for tangential error correction) or may cause radially separated pole segments to act as opposite magnetic poles, depending upon the orientation of the magnet as explained.

It will also be recognized that, while the apparatus has been shown as capable of producing axial or transverse field,,depending upon whether the magnet is perpendicular or parallel to the pole segments, it is also possible to produce a composite field having both axial and trans. verse components by orienting the magnets to positions intermediately to the described positions.

Having thus described our invention, what we claim as .new and desire to secure by Letters Patent is:

1. An adjunct for a cathode-ray tube of the type including a plane of deflection at which electrons are subjected to a scanning deflection in their transit along the axis of such tube toward a mosaic screen made up of a plurality of respectively ditferent elemental areas, said adjunct comprising: a magnet having north and south poles; a first pair of pole segments of magnetic material adapted to be arranged in the region of such screen and spaced from each other axially of such tube; a second pair of pole segments of magnetic material spaced from ,said first pair but in alignment respectively with the segments of said first pair; and means supporting said magnet for movement such that a line between said north and south poles may be varied between parallelism with perpendicularity to said segments.

- 2. The invention as defined by claim 1 wherein said means for supporting said magnet for movement comprises means forrotating said magnet about an axis normal to such line between said north and south poles.

3. The invention as defined by claim 1 wherein said means for supporting said magnet for movement comprises means for rotating said magnet about an axis normal to such line between said north and south poles; and a ring-like member of magnetic material supported coaxially with but spaced from said magnet and capable of short-circuiting flux from said magnet.

,4. An adjunct for a cathode-ray tube of the type including a plane-of deflection at which electrons are subjected to a scanning deflection in their transit along the axis of such tube toward a mosaic screen made up of a plurality of respectively different elemental areas, said adjunct comprising; a magnet having north and south poles; a first pair of pole segments of magnetic material adapted to be arranged in the region of such screen and ,spaced from each other axially of such tube; a second pair of pole segments of magnetic material spaced from said first pair but in alignment respectively with the segments of said first pair; and means supporting said magnet for rotation about an axis normal to said segments such that a line between said north and south poles may be varied between parallelism with perpendicularity to said segments.

5. An adjunct for a cathode-ray tube of the type including a plane of deflection at which electrons are subjected to a scanning deflection in their transitalong the axis of such tube toward a mosaic screen madeup of a plurality of respectively different elemental areas, said adjunct comprising: a magnet having north and south poles; a first pair of pole segments of magnetic material adapted to be arranged in the region of such screen and spaced from each other axially of such tube; a second pair of pole segments of magnetic material spaced from said first pair but in alignment respectively with the segments of said first pair; and means supporting said magnet for movement such that a line between said north and south poles may be varied between parallelism with and 8 perpendicularity to'said segments; and means comprising a magnetic shunt for flux from said magnet.

6. An adjunct for a cathode-ray tube of the type includinga plane of deflection at which electrons are subjected to a scanning deflection in their transit along the axis of such tube toward a mosaic screen made up of a plurality of respectively different elemental area's, said adjunct comprising; a magnet having north and south poles; a first pair of pole segments of magnetic material adapted to be arranged in the region of such screen and spaced from each other axially of such tube; a second pair of pole segments of magnetic material spaced from said first pair but in alignment respectively with the 'segments of said first pair; and means supporting said magnet for movement such that a line between said north and south poles may be varied between parallelism with and perpendicularity to said segments; a magnetic shunt for said magnet; and means for producing relative movement between said shunt and magnet for varying the'effective field produced by said magnet.

7. An adjunct for a cathode-ray tube of the type including a plane of deflection at which electrons are subjected to a scanning deflection in their transit along the axis of such tube toward a mosaic screen made up of a plurality of respectively difierent elemental areas, said adjunct comprising: a plurality of permanent magnets each having a north and south pole; a plurality of pairs of arcuate pole-segments of magnetic material; means for supporting said pole-segments around the screen region of such tube in such relation that the segments of one pair are spaced from each other axially of such tube and the segments of an adjacent pair are spaced from but respectively aligned with the segments of said first pair; and means for supportingone of said magnets in the region between each adjacent pair of segments; and means for producing relative movement between at least one 'of said magnets and its associated pole segments such as to vary the orientation of said magnet from parallelism with to perpendicularity to said segments.

8. The combination with a cathode ray tube ha ving a longitudinal axis and a plane of deflection at which electrons are subjected to a scanning deflection in their transit in angularly related paths to a mosaic screen of the type comprising areas of respectively different characteristics, of a magnet having north and south poles; a first pair of pole segments of magnetic material adapted to be arranged in the region of such screen and spaced from each other axially of such tube; a second pair of pole segments of magnetic material spaced from said first pair but in alignment respectively with the segments .of saidfirst pair; and means supporting said magnet for movement such that a line between said north'an'd south poles may be varied between parallelism with andperpendicularity to said segments.

9. The combination with a cathode ray tube having a longitudinal axis and a plane of deflection at which electrons are subjected to a scanning deflection in their transit in angularly related paths to a mosaic screen of the type comprising areas of respectively different characteristics, of a magnet having north and south poles; a first pair of pole segments of magnetic material adapted to be arranged in the region of such screen and spaced from each other axially of such tube; a second pair of pole segments of magnetic material spaced from said first pair but in alignment respectively with the segments of said first pair; and means supporting said magnet for rotation about an axis normal to said segments such that a line between said north and south poles may be varied between parallelism pole segments of magnetic material adapted to be arranged in the region of such screen and spaced from each other axially of such tube; a second pair of pole segments of magnetic material spaced from said first pair but in alignment respectively with the segments of said first pair; and means supporting said magnet for movement such that a line between said north and south poles may be varied between parallelism with and perpendicularity to said segments; and means comprising a magnetic shunt for flux from said magnet.

11. The combination with a color image-reproducing cathode ray tube having a longitudinal axis and a plane of deflection at which electrons are subjected to a rectangular raster scanning deflection in their transit in angularly related paths through an apertured electrode to a mosaic screen of the type comprising areas of respectively dilferent color characteristics, of a plurality of permanent magnets each having a north and south pole; a plurality of pairs of arcuate pole-segments of magnetic material; means for supporting said pole-segments around the screen region of such tube in such relation that the segments of one pair are spaced from each other axially of such tube and the segments of an adjacent pair are spaced from but respectively aligned with the segments of said first pair; and means for supporting one of said magnets in the region between each adjacent pair of segments; and means for producing relative movement between at least one of said magnets and adjunct comprising: a first pair of pole segments of mag-- netic material adapted to be arranged in the region of such screen and spaced from each other axially of such tube; a second pair of pole segments of magnetic material spaced from said first pair but in alignment respectively with the segments of said first pair; and magnet means operative in the space between such first and second pairs of pole segments for selectively energizing either the segments of one pair as magnetic poles opposite from the segments of the second pair or the aligned segments of said first and second pairs as magnetic pole I opposite from the other segments of said first and second pairs.

References Cited in the file of this patent UNITED STATES PATENTS 2,541,446 Trott Feb. 13, 1951 2,511,003 Friend Sept. 16, 1952 2,677,779 Goodrich May 4, 1954 

